1. Field of the Invention
The present invention relates to a shift mechanism for an automatic transmission and, more particularly, to a structural combination of a frictional engagement element, a planetary gear unit, a hydraulic servo unit and a one-way clutch.
2. Related Art
In an automatic transmission having a one-way clutch of the free wheel type, one element of a planetary gear unit is disengageably fixed to the case for a shift, in dependence upon the direction of rotation, and is arranged in parallel with a multi-disc brake so that it may establish engine braking when reversely driven. Specifically, a one-way clutch is arranged to have one of its inner and outer races fixed to the case and the other race connected to one element of the planetary gear unit. Likewise, the brake has one frictional engagement element set connected to the case and its other set connected to one element of the planetary gear unit.
In the prior art, in which the one-way clutch and the brake are combined, the one-way clutch has its outer race connected to the carrier of the planetary gear unit to allow the frictional engagement element to be located between the carrier and the case, and has its inner race fixed to the case, as disclosed in Japanese Patent Laid-Open No. 288753/1987. In Japanese Patent Laid-Open No. 126147/1993, on the other hand, there is disclosed another arrangement, in which the one-way clutch has its inner race connected to the ring gear, to allow the frictional engagement element to be located between the ring gear and the case, and its outer race fixed to the case.
In the former type in which the inner race is fixed to the case, the inner race does not rotate, which makes it impossible to use the ordinary lubrication means, involving applying a centrifugal force to the lubricating oil fed from the in-shaft oil passage, to introduce centrifugal oil pressure into the one-way clutch. Thus, there arises the problem that another lubricating oil passage must be formed leading from the case to the inside of the one-way clutch. On the other hand, in a construction having the outer race fixed to the case, the centrifugal oil pressure can be fed into the one-way clutch through a radial lubricating oil passage formed in the inner race and, as a result, the lubrication can be effected by ordinary means of simplified construction. However, the outer race is axially interposed between the piston of the hydraulic servo unit and the frictional engagement element. This makes it necessary to adopt a construction in which the frictional engagement element is axially pressed by a forked member extending from the piston through a hole extending through the outer race, so that the pressure received at the face of the friction member is uneven. In order to compensate for this latter problem, therefore, the operating force must be increased by enlarging the size of the hydraulic servo unit, but this, in turn, decreases the durability of the friction member, thus creating another problem.